History of Earth
See also: Timeline of Earth The geological history of Earth follows the major events in Earth's past based on the , a system of based on the study of the planet's rock layers ( ). Earth formed by accretion from the , a disk-shaped mass of dust and gas left over from the formation of the Sun, which also created the rest of the . Earth was initially molten due to extreme and frequent collisions with other bodies. Eventually, the outer layer of the planet cooled to form a solid when water began accumulating in the atmosphere. The formed soon afterwards, possibly as a result of the impact of a planetoid with the Earth. and volcanic activity produced the primordial atmosphere. Condensing , augmented by ice delivered from s, . As the surface continually reshaped itself over hundreds of millions of years, continents formed and broke apart. They , occasionally combining to form a . Roughly , the earliest-known supercontinent , began to break apart. The continents later recombined to form , , then finally , which broke apart . The present pattern of s began about , then intensified at the end of the . The polar regions have since undergone repeated cycles of glaciation and thaw, repeating every 40,000–100,000 years. The of the ended about 10,000 years ago. Precambrian The Precambrian includes approximately 90% of geologic time. It extends from 4.6 billion years ago to the beginning of the Cambrian Period (about 541 ). It includes three eons, the Hadean, Archean, and Proterozoic. Major volcanic events altering the Earth's environment and causing extinctions may have occurred 10 times in the past 3 billion years. Hadean Eon }} During Hadean time (4.6–4 ), the was forming, probably within a large cloud of gas and dust around the sun, called an from which . The Hadean Eon is not formally recognized, but it essentially marks the era before we have adequate record of significant solid rocks. The oldest dated s date from about . was initially molten due to extreme and frequent collisions with other bodies. Eventually, the outer layer of the planet cooled to form a solid when water began accumulating in the atmosphere. The formed soon afterwards, possibly as a result of the of a large planetoid with the Earth. Some of this object's mass merged with the Earth, significantly altering its internal composition, and a portion was ejected into space. Some of the material survived to form an orbiting moon. More recent potassium isotopic studies suggest that the Moon was formed by a smaller, high-energy, high-angular-momentum giant impact cleaving off a significant portion of the Earth. Outgassing and activity produced the primordial atmosphere. Condensing , augmented by ice delivered from s, . During the Hadean the occurred (approximately ) during which a large number of impact craters are believed to have formed on the Moon, and by inference on Earth, , and as well. Archean Eon The Earth of the early Archean ( ) may have had a different tectonic style. During this time, the Earth's cooled enough that rocks and continental plates began to form. Some scientists think because the Earth was hotter, that plate tectonic activity was more vigorous than it is today, resulting in a much greater rate of recycling of crustal material. This may have prevented cratonisation and continent formation until the cooled and convection slowed down. Others argue that the subcontinental lithospheric mantle is too buoyant to and that the lack of Archean rocks is a function of and subsequent events. In contrast to the , Archean rocks are often heavily metamorphized deep-water sediments, such as s, s, volcanic sediments and s. s are typical Archean formations, consisting of alternating high- and low-grade metamorphic rocks. The high-grade rocks were derived from volcanic s, while the low-grade metamorphic rocks represent deep-sea sediments eroded from the neighboring island rocks and deposited in a . In short, greenstone belts represent sutured protocontinents. The Earth's was established 3.5 billion years ago. The flux was about 100 times the value of the modern , so the presence of the magnetic field helped prevent the planet's atmosphere from being stripped away, which is what probably happened to the . However, the field strength was lower than at present and the was about half the modern radius. Proterozoic Eon The geologic record of the Proterozoic ( ) is more complete than that for the preceding . In contrast to the deep-water deposits of the Archean, the Proterozoic features many that were laid down in extensive shallow ; furthermore, many of these rocks are less than Archean-age ones, and plenty are unaltered. Study of these rocks show that the eon featured massive, rapid al accretion (unique to the Proterozoic), s, and wholly modern activity. Roughly , the earliest-known supercontinent , began to break apart. The continents later recombined to form , 600–540 Ma. The first-known glaciations occurred during the Proterozoic, one began shortly after the beginning of the eon, while there were at least four during the Neoproterozoic, climaxing with the of the Varangian glaciation. Phanerozoic Eon The Phanerozoic Eon is the current eon in the geologic timescale. It covers roughly 541 million years. During this period continents drifted about, eventually collected into a single landmass known as and then split up into the current continental landmasses. The Phanerozoic is divided into three eras – the , the and the . Most of biological evolution occurred during this time period. Paleozoic Era The Paleozoic spanned from roughly (Ma) and is subdivided into six ; from oldest to youngest they are the , , , , and . Geologically, the starts shortly after the breakup of a supercontinent called and at the end of a global ice age. Throughout the early Paleozoic, the Earth's landmass was broken up into a substantial number of relatively small continents. Toward the end of the era the continents gathered together into a supercontinent called , which included most of the Earth's land area. Cambrian Period The Cambrian is a major division of the that begins about 541.0 ± 1.0 Ma. continents are thought to have resulted from the breakup of a supercontinent called Pannotia. The waters of the Cambrian period appear to have been widespread and shallow. Continental drift rates may have been anomalously high. , and remained independent continents following the break-up of the supercontinent of Pannotia. started to drift toward the South Pole. covered most of the southern hemisphere, and minor oceans included the , and . Ordovician period The Ordovician period started at a major extinction event called the some time about 485.4 ± 1.9 Ma. During the the southern continents were collected into a single continent called Gondwana. Gondwana started the period in the equatorial latitudes and, as the period progressed, drifted toward the South Pole. Early in the Ordovician the continents Laurentia, Siberia and Baltica were still independent continents (since the break-up of the supercontinent Pannotia earlier), but began to move toward Laurentia later in the period, causing the Iapetus Ocean to shrink between them. Also, broke free from Gondwana and began to head north toward Laurentia. The was formed as a result of this. By the end of the period, Gondwana had neared or approached the pole and was largely glaciated. The Ordovician came to a close in a series of s that, taken together, comprise the second-largest of the five major extinction events in in terms of percentage of that became extinct. The only larger one was the Permian-Triassic extinction event. The extinctions occurred approximately and mark the boundary between the Ordovician and the following Period. The most-commonly accepted theory is that these events were triggered by the onset of an , in the Hirnantian faunal stage that ended the long, stable conditions typical of the Ordovician. The ice age was probably not as long-lasting as once thought; study of oxygen s in fossil brachiopods shows that it was probably no longer than 0.5 to 1.5 million years. The event was preceded by a fall in atmospheric carbon dioxide (from 7000ppm to 4400ppm) which selectively affected the shallow seas where most organisms lived. As the southern supercontinent drifted over the South Pole, ice caps formed on it. Evidence of these ice caps have been detected in Upper Ordovician rock strata of North Africa and then-adjacent northeastern South America, which were south-polar locations at the time. Silurian Period The Silurian is a major division of the that started about 443.8 ± 1.5 Ma. During the , Gondwana continued a slow southward drift to high southern latitudes, but there is evidence that the Silurian ice caps were less extensive than those of the late Ordovician glaciation. The melting of ice caps and glaciers contributed to a rise in s, recognizable from the fact that Silurian sediments overlie eroded Ordovician sediments, forming an . Other s and continent fragments drifted together near the equator, starting the formation of a second supercontinent known as . The vast ocean of Panthalassa covered most of the northern hemisphere. Other minor oceans include Proto-Tethys, Paleo-Tethys, Rheic Ocean, a seaway of Iapetus Ocean (now in between Avalonia and Laurentia), and newly formed . Devonian Period The Devonian spanned roughly from 419 to 359 Ma. The period was a time of great tectonic activity, as and Gondwana drew closer together. The continent Euramerica (or Laurussia) was created in the early Devonian by the collision of Laurentia and Baltica, which rotated into the natural dry zone along the . In these near-deserts, the sedimentary beds formed, made red by the oxidized iron ( ) characteristic of drought conditions. Near the equator Pangaea began to consolidate from the plates containing North America and Europe, further raising the northern and forming the in and . The southern continents remained tied together in the supercontinent of . The remainder of modern Eurasia lay in the Northern Hemisphere. Sea levels were high worldwide, and much of the land lay submerged under shallow seas. The deep, enormous Panthalassa (the "universal ocean") covered the rest of the planet. Other minor oceans were Paleo-Tethys, Proto-Tethys, Rheic Ocean and Ural Ocean (which was closed during the collision with Siberia and Baltica). Carboniferous Period The Carboniferous extends from about 358.9 ± 0.4 to about 298.9 ± 0.15 Ma. A global drop in sea level at the end of the Devonian reversed early in the ; this created the widespread epicontinental seas and carbonate deposition of the . There was also a drop in south polar temperatures; southern Gondwana was glaciated throughout the period, though it is uncertain if the ice sheets were a holdover from the Devonian or not. These conditions apparently had little effect in the deep tropics, where lush swamps flourished within 30 degrees of the northernmost glaciers. A mid-Carboniferous drop in sea-level precipitated a major marine extinction, one that hit s and s especially hard. This sea-level drop and the associated unconformity in North America separate the from the . The Carboniferous was a time of active mountain building, as the supercontinent Pangea came together. The southern continents remained tied together in the supercontinent Gondwana, which collided with North America-Europe ( ) along the present line of eastern . This continental collision resulted in the in Europe, and the in North America; it also extended the newly uplifted Appalachians southwestward as the . In the same time frame, much of present eastern plate welded itself to Europe along the line of the . There were two major oceans in the Carboniferous the Panthalassa and Paleo-Tethys. Other minor oceans were shrinking and eventually closed the (closed by the assembly of South and North America), the small, shallow (which was closed by the collision of , and continents, creating the Ural Mountains) and Proto-Tethys Ocean. Permian Period The Permian extends from about 298.9 ± 0.15 to 252.17 ± 0.06 Ma. During the all the Earth's major land masses, except portions of East Asia, were collected into a single supercontinent known as . Pangaea straddled the equator and extended toward the poles, with a corresponding effect on ocean currents in the single great ocean ( , the universal sea), and the , a large ocean that was between Asia and Gondwana. The Cimmeria continent rifted away from Gondwana and drifted north to Laurasia, causing the Paleo-Tethys to shrink. A new ocean was growing on its southern end, the Tethys Ocean, an ocean that would dominate much of the Mesozoic Era. Large continental landmasses create climates with extreme variations of heat and cold ("continental climate") and monsoon conditions with highly seasonal rainfall patterns. s seem to have been widespread on Pangaea. Mesozoic Era }} }} The Mesozoic extended roughly from . After the vigorous convergent plate mountain-building of the late , tectonic deformation was comparatively mild. Nevertheless, the era featured the dramatic rifting of the supercontinent . Pangaea gradually split into a northern continent, , and a southern continent, . This created the that characterizes most of the coastline (such as along the U.S. East Coast) today. Triassic Period The Triassic Period extends from about 252.17 ± 0.06 to 201.3 ± 0.2 Ma. During the , almost all the Earth's land mass was concentrated into a single centered more or less on the equator, called ("all the land"). This took the form of a giant " " with an east-facing "mouth" constituting the , a vast gulf that opened farther westward in the mid-Triassic, at the expense of the shrinking , an ocean that existed during the . The remainder was the world-ocean known as ("all the sea"). All the deep-ocean sediments laid down during the Triassic have disappeared through of oceanic plates; thus, very little is known of the Triassic open ocean. The supercontinent Pangaea was rifting during the Triassic—especially late in the period—but had not yet separated. The first nonmarine sediments in the that marks the initial break-up of Pangea—which separated from —are of Late Triassic age; in the U.S., these thick sediments comprise the . Because of the limited shoreline of one super-continental mass, Triassic marine deposits are globally relatively rare; despite their prominence in , where the Triassic was first studied. In , for example, marine deposits are limited to a few exposures in the west. Thus Triassic is mostly based on organisms living in lagoons and hypersaline environments, such as Estheria crustaceans and terrestrial vertebrates. Jurassic Period The Jurassic Period extends from about 201.3 ± 0.2 to 145.0 Ma. During the early , the supercontinent broke up into the northern supercontinent and the southern supercontinent ; the opened in the new rift between North America and what is now 's . The Jurassic North was relatively narrow, while the South Atlantic did not open until the following Cretaceous Period, when Gondwana itself rifted apart. The closed, and the basin appeared. Climates were warm, with no evidence of . As in the Triassic, there was apparently no land near either pole, and no extensive ice caps existed. The Jurassic geological record is good in western , where extensive marine sequences indicate a time when much of the continent was submerged under shallow tropical seas; famous locales include the and the renowned late Jurassic n of and . In contrast, the North American Jurassic record is the poorest of the Mesozoic, with few outcrops at the surface. Though the left marine deposits in parts of the northern plains of the and during the late Jurassic, most exposed sediments from this period are continental, such as the deposits of the . The first of several massive s were emplaced in the northern beginning in the mid-Jurassic, marking the . Important Jurassic exposures are also found in Russia, India, South America, Japan, and the United Kingdom. Cretaceous Period period}} The Cretaceous Period extends from circa to . During the , the late -early Mesozoic of completed its breakup into present day s, although their positions were substantially different at the time. As the widened, the convergent-margin that had begun during the Jurassic continued in the , as the was followed by the and . Though Gondwana was still intact in the beginning of the Cretaceous, itself broke up as , and rifted away from (though and remained attached to each other); thus, the South Atlantic and s were newly formed. Such active rifting lifted great undersea mountain chains along the welts, raising worldwide. To the north of Africa the continued to narrow. Broad shallow seas advanced across central (the ) and Europe, then receded late in the period, leaving thick marine deposits sandwiched between beds. At the peak of the Cretaceous , one-third of Earth's present land area was submerged. The Cretaceous is justly famous for its ; indeed, more chalk formed in the Cretaceous than in any other period in the . activity—or rather, the circulation of seawater through the enlarged ridges—enriched the oceans in calcium; this made the oceans more saturated, as well as increased the bioavailability of the element for . These widespread s and other make the Cretaceous rock record especially fine. Famous from North America include the rich marine fossils of 's and the terrestrial fauna of the late Cretaceous . Other important Cretaceous exposures occur in and . In the area that is now India, massive beds called the were laid down in the very late Cretaceous and early Paleocene. Cenozoic Era The Cenozoic Era covers the million years since the up to and including the present day. By the end of the era, the continents had rifted into nearly their present form. became and , while split into , , , and the , which collided with the Asian plate. This impact gave rise to the Himalayas. The Tethys Sea, which had separated the northern continents from Africa and India, began to close up, forming the . Paleogene Period The Paleogene (alternatively Palaeogene) is a unit of that began and ended 23.03 Ma and comprises the first part of the Era. This period consists of the , and Epochs. Paleocene Epoch The Paleocene, lasted from to . In many ways, the continued processes that had begun during the late Cretaceous Period. During the Paleocene, the s continued to drift toward their present positions. Supercontinent had not yet separated into three continents. and were still connected. and were still intermittently joined by a land bridge, while Greenland and North America were beginning to separate. The of the late Cretaceous continued to uplift the in the American west, which ended in the succeeding epoch. South and North America remained separated by equatorial seas (they joined during the ); the components of the former southern supercontinent continued to split apart, with , South America, and pulling away from each other. Africa was heading north toward , slowly closing the , and began its migration to Asia that would lead to a tectonic collision and the formation of the . Eocene Epoch During the ( - ), the continents continued to drift toward their present positions. At the beginning of the period, Australia and Antarctica remained connected, and warm ial currents mixed with colder Antarctic waters, distributing the heat around the world and keeping global temperatures high. But when Australia split from the southern continent around 45 , the warm equatorial currents were deflected away from Antarctica, and an isolated cold water channel developed between the two continents. The Antarctic region cooled down, and the ocean surrounding Antarctica began to freeze, sending cold water and ice floes north, reinforcing the cooling. The present pattern of s began about . The northern of began to break up, as , and drifted apart. In western North America, started in the Eocene, and huge lakes formed in the high flat basins among uplifts. In Europe, the finally vanished, while the uplift of the isolated its final remnant, the , and created another shallow sea with island s to the north. Though the North was opening, a land connection appears to have remained between North America and Europe since the faunas of the two regions are very similar. continued its journey away from and began its collision with , creating the n orogeny. Oligocene Epoch The Oligocene Epoch extends from about to . During the the continents continued to drift toward their present positions. continued to become more isolated and finally developed a permanent . in western continued, and the started to rise in as the continued to push north into the , isolating the remnants of . A brief marine incursion marks the early Oligocene in Europe. There appears to have been a land bridge in the early Oligocene between and since the of the two regions are very similar. During the Oligocene, was finally detached from and drifted north toward . It also allowed the to flow, rapidly cooling the continent. Neogene Period The Neogene Period is a unit of starting 23.03 Ma. and ends at 2.588 Ma. The Neogene Period follows the Period. The Neogene consists of the and and is followed by the Period. Miocene Epoch The Miocene extends from about 23.03 to 5.333 Ma. During the continents continued to drift toward their present positions. Of the modern geologic features, only the land bridge between and was absent, the subduction zone along the margin of South America caused the rise of the and the southward extension of the peninsula. continued to collide with . The Tethys Seaway continued to shrink and then disappeared as collided with in the - n region between 19 and 12 ( 2004). Subsequent uplift of mountains in the western region and a global fall in sea levels combined to cause a temporary drying up of the Mediterranean Sea resulting in the near the end of the Miocene. Pliocene Epoch The Pliocene extends from to . During the continents continued to drift toward their present positions, moving from positions possibly as far as from their present locations to positions only 70 km from their current locations. South America became linked to North America through the during the Pliocene, bringing a nearly complete end to South America's distinctive faunas. The formation of the Isthmus had major consequences on global temperatures, since warm equatorial ocean currents were cut off and an Atlantic cooling cycle began, with cold Arctic and Antarctic waters dropping temperatures in the now-isolated Atlantic Ocean. 's collision with formed the , cutting off the remnants of the . Sea level changes exposed the land-bridge between and Asia. Near the end of the Pliocene, about (the start of the Quaternary Period), the began. The polar regions have since undergone repeated cycles of glaciation and thaw, repeating every 40,000–100,000 years. Quaternary Period Pleistocene Epoch The Pleistocene extends from to 11,700 years before present. The modern s were essentially at their present positions during the , the upon which they sit probably having moved no more than relative to each other since the beginning of the period. Holocene Epoch The Holocene Epoch began approximately 11,700 calendar years before present and continues to the present. During the , continental motions have been less than a kilometer. The of the ended about 10,000 years ago. Ice melt caused world about in the early part of the Holocene. In addition, many areas above about latitude had been depressed by the weight of the Pleistocene s and rose as much as over the late Pleistocene and Holocene, and are still rising today. The sea level rise and temporary land depression allowed temporary marine incursions into areas that are now far from the sea. Holocene marine fossils are known from , , and . Other than higher latitude temporary marine incursions associated with glacial depression, Holocene fossils are found primarily in lakebed, floodplain and cave deposits. Holocene marine deposits along low-latitude coastlines are rare because the rise in sea levels during the period exceeds any likely upthrusting of non-glacial origin. in resulted in the emergence of coastal areas around the , including much of . The region continues to rise, still causing weak s across . The equivalent event in North America was the rebound of , as it shrank from its larger, immediate post-glacial phase, to near its present boundaries. References Category:History of the world